In add-on script, how to automatically rewrite string in the bottom statusbar on each edge select/deselect in edit mode
I only answer the second one for you since it is a little bit hard to find. But it is actually very useful in many case.

status_text_set(string)

status_text_set(text) in bpy.types.WorkSpace(ID):


  classmethod status_text_set(text)  
  
  Set the status bar text, typically key shortcuts for modal operators
  
  
  Parameters:    text (string) – Text, New string for the status bar, None clears the text
  


So we can bpy.types.WorkSpace.status_text_set("hello world") to set the status bar, cool.

Or since context got an workspace as well, use context.workspace.status_text_set("foo") will be a better idea when using in operator or function which has it own context. - @batFINGER



Important

Clear the content by assign it a None, or it will keep showing there until next function call. You should consider clear it after you leave your operator.